You do have a defence as you were not the keeper at the time of the contraventions.
However, you do have to raise evidence that you were not the keeper.
the insurance is not really a proper demand but the sales receipt is I'm afraid.
If you cannot do that then you could rely on the letter from the DVLA but whether that would be sufficient depends.
The local authority will probably not accept that but you could appeal to the Adjudicator who may.
I suppose you could raise other evidence of the transfer - money into your account etc.
Can I clarify anything for you?
All the best.